The present invention relates to a liquid crystal display panel. More particularly, the present invention relates to a liquid crystal display panel including nematic liquid crystal twisted at a high degree which is arranged between a pair of substrates having a transparent electrode and an orientation plane and which has a positive anisotropy of refractive index.
In order to provide a multiplex-driven simple matrix type liquid crystal display panel having a larger capacity and higher image quality, a liquid crystal cell having a steep threshold voltage characteristic is required. It has been recently discovered that the threshold voltage characteristic of a liquid crystal cell can be significantly improved by increasing the twist angle of a cell structure having twisted orientation used in a conventional twisted nematic (TN) mode. Then, new display modes utilizing this effect have been developed one after another. Those new modes include SBE (super-twisted birefringence effect), STN (super-twisted nematic) and OMI (optical mode interference) modes.
The SBE mode is characterized in that the twist angle is set at 270.degree. and in that the pretilt angle of liquid crystal molecules is as large as about 20.degree.. The STN mode is characterized in that the pretilt angle is small and the twist angle is set at 180.degree. as disclosed in Japanese unexamined patent publication (KOKAI) No. S60-50511, for example. The OMI mode is characterized by an improvement wherein a coloration phenomenon in an off-state of the STN mode is avoided to provide a substantially achromatic color. These modes are rapidly coming in practice as highly multiplex-driven and high quality liquid crystal displays that replace the conventional TN mode.
Cells in the SBE, STN and OMI modes have a structure wherein a cell having twisted orientation is sandwiched by two polarizers like a conventional TN cell as shown in FIG. 4. Major structural differences between those display modes exist in the values of the twist angle .PHI. of liquid crystal molecules, retardation .DELTA.n.multidot.d (the product of the anisotropy of refractive index of liquid crystal and a cell gap), the pretilt angle .theta..sub.0 and the azimuth angles .beta. and .gamma. of the polarizers shown in FIG. 9. Typical values of these parameters are shown in FIG. 5.
The STN mode will be explained below in more detail. In the STN mode, a pair of polarizers were provided above and below the liquid crystal cell and an included angle made by their polarizing axis and a molecular axis of liquid crystal molecules adjacent to the electrode substrate was in a range of 30.degree. to 60.degree.. Due to that, coloration is caused by birefringence and a hue of appearance is not white even if no voltage is applied to the liquid crystal cell, presenting a hue of green to yellowish red color in general. The hue of the appearance generally turns blue, not black, in a state when a voltage is applied.
FIG. 3 is a typical view of the conventional STN mode liquid crystal display panel. In the figure, the liquid crystal cell is sandwiched by a pair of upper and lower polarizers 9 and 10. A transparent electrode 5 composed of ITO or the like is patterned on the inner surface of an upper substrate 1 and an orientation film 6 is applied thereon and is rubbed. similarly, a transparent electrode 7 is patterned on the inner surface of a lower substrate 2 and an orientation film 8 is applied thereon and is rubbed. The upper and lower substrates 1 and 2 face from each other via a sealing material 3, thus holding liquid crystal 4. The liquid crystal 4 is twistedly oriented at a twist angle of about 180.degree. to 200.degree. for example.
In the conventional liquid crystal display panel, normally white and normally black modes may be obtained by selecting a direction of a polarizing axis of the pair of upper and lower polarizers 9 and 10. The normally white mode presents white color when no voltage is applied (off state) and presents black color when a voltage is applied (on state). Contrary to that, the normally black mode presents black color in the off-state and presents white color in the on-state. However, in the normally white mode, the appearance color of the conventional liquid crystal display panel actually assumes green, yellowish green, yellow or yellowish red in the off-state and blue or dark blue in the on-state. in the normally black mode, it turns dark blue in the off-state and yellow in the on-state.
Those colors are not preferable in general as colors displayed on the display panel. A combination of white and black is most suitable psychologically and physically as displayed colors after all and a liquid crystal display panel capable of displaying white and black is requested. In particular, it exerts a large influence on a vividness of color in displaying in color in combination with color filters.
It should be noted that an arrangement in which a compensating optically anisotropic substance is. used as means for eliminating such coloration as described above has been proposed and has been disclosed in Japanese examined patent publication (KOKOKU) No. Hei. 3-50249 for example. FIG. 6 schematically shows the arrangement disclosed therein. In the figure, linear polarizers 102 and 103 are disposed before and after a liquid crystal cell 101. An optically anisotropic substance 120 is interposed between the liquid crystal cell 101 and the linear polarizer 103. As the optically anisotropic substance 120, a liquid crystal component, an uniaxial oriented film, a liquid crystal polymer film, a film created by a mixture of liquid crystal and a polymer compound or the like is used.
Referring still to FIG. 6, a compensating effect of the optically anisotropic substance 120 will be explained briefly.
In the figure, an incident beam 130 is generally white light and includes lights of all wavelengths in the visible region. Its polarization direction is also at random. Passing through the linear polarizer 102, the incident light 130 results in a set of linearly polarized beams 130B, 130G and 130R whose polarizing directions are uniform. 130B, 130G and 130R here refer to polarized beams having 450 nm, 550 nm and 650 nm of wavelength, respectively. Although linearly polarized beams have wavelengths other than those that are naturally contained therein, only typical wavelengths of three colors of blue (B), green (G) and red (R) are shown here. Then, those linearly polarized beams 130B, 130G and 130R pass through the liquid crystal cell 101. The liquid crystal layer within the liquid crystal cell has a structure in which nematic liquid crystal which presents an uniaxial anisotropy of refractive index optically is twisted. Passing through the liquid crystal layer having such structure, the linearly polarized beams 130B, 130G and 130R are polarized as denoted by 131B, 1310 and 131R, respectively. When they pass through the liquid crystal layer like that, wavelength dispersion occurs in the polarization state. If the optically anisotropic substance 120 is not interposed for instance, those polarized beams 131B, 131G and 131R pass directly through the linear polarizer 103. Then, only components of the polarized beams having respective wavelengths which correspond to the direction of the linear polarizer 103 pass through it. Because that component differs per 131B 131G and 131R, a combined result does not turn white color and is colored.
Thus, the wavelength dispersion was caused by the birefringence and the coloration was unavoidable in the conventional STN mode liquid crystal cell. Then, the optically anisotropic substance 120 is interposed to cancel the wavelength dispersion caused when the polarized beams 131B, 131G and 131R pass through the liquid crystal cell 101. Each polarized beam component 132B, 132G and 132R which has passed through the optically anisotropic substance 120 is polarized almost linearly and passes through the polarizer 103 as it is. The polarized beam components 132B, 132G and 132R which have passed through the linear polarizer 103 are combined as they are, presenting the same white color with the incident beam.
The coloring phenomenon caused by the wavelength dispersion described above may be considered as a shift of color caused by the thickness of the liquid crystal layer when the beam passes through the liquid crystal cell. Furthermore, a shift of color is caused by twisted orientation of liquid crystal molecules. The beam is influenced by both of them. The uniaxial oriented film such as polycarbonate is used as described above as a compensating material for recovering the shift. However, although it may compensate the shift caused by the thickness of the liquid crystal layer, it is impossible to compensate the shift of color caused by the twisted orientation of the liquid crystal molecules. it then causes a leakage of beams of a wide range of wavelengths, causing coloration. In order to solve that problem, a compensating material having a twisted structure has been developed recently and has allowed the compensation of both shifts. Because it can completely shut down light, a contrast of white and black become clearer and because it can enhance the transmittance, it contributes a reduction in power consumption.
FIG. 7 shows the compensating material having the twisted structure. As shown in the figure, the compensating material comprises polymer films and about 3000 layers of polymer liquid crystal 202 are laminated within a thickness of only 3 .mu.m on a substrate 201. The laminated layers of the polymer liquid crystal 202 are twisted by 240.degree.. The shift of color caused by the twist of the liquid crystal molecules, in addition to the shift of color caused by the thickness of the liquid crystal cell, may be compensated and varied by laminating the polymer films in a relationship reverse to the twisted orientation of the liquid crystal cell.
In the SBE and STN modes, since the orientation of molecules on the wall surface and the polarizing direction of the polarizers are shifted as shown in FIGS. 4 and 5, an incident polarized beam is split into beams having planes of polarization which are parallel with and vertical to the orientation of the molecules (abnormal beam and normal beam, respectively). Since these beams propagate in liquid crystal at different rates, they interfere with each other when passing through the upper polarizer. The conditions of this interference significantly vary against slight changes in the molecular orientation, resulting in a steep threshold characteristic. Thus, in the SBE and STN modes, the retardation .DELTA.n.multidot.d and the azimuth angles .beta. and .gamma. of polarizers are optimized to provide better display characteristics by means of a sophisticated combination of the steep transformation of the molecular orientation as a result of the application of a voltage and an optical birefringence effect. In the case of the SBE mode, the value of .DELTA.n.multidot.d is about 0.8 .mu.m. There are two types of arrangements of azimuth angles .beta. and .gamma. of polarizers, i.e., an arrangement wherein .beta. and .gamma. are about 30.degree. and 60.degree., respectively, and an arrangement wherein .beta. and .gamma. are about 30.degree. and -30.degree., respectively. Under the former condition, the displayed color will be yellow in an unselected state and black in a selected state, which is referred to as a "yellow mode".
On the other hand, under the latter condition, the displayed color will be blue in the unselected state and substantially achromatic in the selected state, which is referred to as a "blue model". The SBE and STN modes have problems in that it is difficult to provide multicolor display because the coloration in the unselected state occurs as a result of the birefringence effect and in that the cell gap must be controlled with an accuracy as high as +0.1 .mu.m to 0.2 .mu.m or less because variations in the cell gap are sensitively reflected as changes in the birefringent color. As a technique for preventing coloring, methods utilizing polymer films which are an optically anisotropic substance have been proposed, one example of such methods being disclosed in Japanese examined patent publication (KOKOKU) No. Hei. 3-50249. As described above, in the conventional SBE and STN modes, the colored state has been unavoidable because of wavelength dispersion as a result of birefringence. The optically anisotropic substance has an effect of canceling such wavelength dispersion.
FIGS. 8a through 8c schematically show the effect of the polymer film which is the optically anisotropic substance for preventing coloring. Hereinafter, the polymer film may be referred to as an RCF (Retardation Compensation Film). As shown in the figure, the RCF is basically uniaxial and has a positive anisotropy of refractive index. Axes of coordinate orthogonal from each other along plane directions are denoted by x and y axes and that in the thickness direction is denoted by z-axis. When a refractive index in the x-axis direction is nx, a refractive index in the y-axis direction is ny and a refractive index in the z-axis direction is nz, a relationship of ny&gt;nx=nz holds. That is, the RCF has an equal refractive index in biaxial directions of x and z and has a refractive index which is different only in the uniaxial direction of y. That is, the RCF has an uniaxial anisotropy of refractive index. This one axis is the y-axis and is referred to as an optical axis hereinafter. Because the refractive index in the direction of the optical axis is larger than the refractive index in the direction orthogonal to the optical axis, the RCF has a positive anisotropy of refractive index. Note that the optical axis (y-axis) is fixed in the case of the RCF 120 used in the first prior art example shown in FIG. 6. In contrast, the optical axis (y-axis) of the RCF of the second prior art example shown in FIG. 7 is twisted along the thickness direction (z-axis direction).
Meanwhile, the liquid crystal molecules within the liquid crystal cell (hereinafter referred to as LC) is uniaxial and shows a positive anisotropy of refractive index optically as described before. FIG. 8b schematically shows it. The LC has an optical axis of x-axis and has a large refractive index nx. It also has small refractive indices ny and nz with respect to y-axis and z-axis directions. Their relationship may be represented by nx&gt;ny=nz. Note that the optical axis (x-axis) is actually twisted along the z-axis direction within the LC.
In the case of an SBE cell, the pretilt angle must be as large as about 20.degree. in order to prevent the occurrence of another orientation-transformed mode wherein the screw axis is tilted. This has not been put into practical use because it requires a rhombic deposition orientation process which is not suitable for mass production. In order to solve this problem, efforts are being put on the development of surface orientation processes which enable larger pretilt angles. From this point of view, an STN mode wherein the twist angle is about 180.degree. to 240.degree. are currently in practical use. However, this STN mode has problems not only in that the use of polymer films which are an optically anisotropic substance can not completely eliminate coloration but also in that it has a poor viewing angle characteristic that results in significant coloration depending on viewing angles.
FIG. 8c shows a state in which the RCF and the LC are superposed. Note that in this case, they are superposed so that the optical axis (y-axis) of the RCF crosses with the optical axis (x-axis) of the LC at right angles. As a result, the refractive index combined with respect to the x-axis direction and the refractive index combined with respect to the y-axis direction becomes almost equal and a symmetrical configuration is brought about with respect to the plane directions. That is, a two-dimensional symmetry is provided and an optically improved display may be obtained as far as it goes. However, a three-dimensional symmetry is not obtained because the refractive index combined with respect to the z-axis direction (thickness direction) is different from that of the x-axis a nd y-axis directions. In other words, they cannot be compensated and varied triaxial-symmetrically. Specifically, the z-axis direction (thickness direction) is related with the viewing angle dependency. Accordingly, although it is possible to eliminate the coloring to a certain degree by using the RCF in the conventional liquid crystal display panel, the contrast changes significantly depending on a viewing angle, damaging a visibility of the display screen.
On the other hand, the basic cell structure in the OMI mode is the same as the STN cell. It is different from the STN mode in that the retardation .DELTA.n.multidot.d of the cell is reduced and in that the azimuth angles of the polarizers are optimized to achieve an elliptic polarization mode which is close to circular polarization. This reduces dependence on wavelength and provides a substantially achromatic color in the unselected state. As a result, black and white display has been substantially achieved to substantially improve the visibility of display and the possibility of multicolor display has been provided. For example, such an OMI mode is disclosed in Japanese unexamined patent publication (KOKAI) No. S63-74030. Another advantage of the OMI mode lies in that the requirement for the accuracy of cell gap control is relaxed. On the other hand, there are problems in that the sharpness of the threshold characteristic is poorer than that achieved in an STN cell and in that contrast is relatively poor because the white level is dark due to low transmittance. Current efforts toward the improvement on these problems are put on increasing the twist angle .PHI. to about 240.degree. and on compensating transmitted spectra by adding a dichroic pigment, although not successful yet.